1. Field of the Invention
The present invention relates to a recording method for recording a signal on an optical information recording medium, primarily an optical disk to and from which optical information can be recorded and read at high speed and high density using a leaser beam or other optical source.
2. Description of the Prior Art
Technologies which apply laser beams to read and/or write high density information are commonly known, and are primarily used with optical disks. Optical disks can be classified into three broad categories: read-only, write-once read-many, and rewritable. Read-only disks include compact disks (CD), mainly used for recording musical information, and video disks (LVD), mainly used for recording image information. With these media, the signal is pre-recorded to the optical disk, and the user can playback the music or video information but is unable to record any additional signals. Recent research has therefore concentrated on the development of a rewritable type of media and drive which enables free and repeated writing and erasure of the signal.
Rewritable types use a recording thin film in which a reversible change between two states is induced by changing the emission conditions of the laser beam or other light source; the principal types of thin films used are magneto-optical and phase change media. Magneto-optical types use a ferromagnetic thin film as the recording thin film, and a signal is recorded by changing the orientation of the magnetic domain. Phase change types principally use a tellurium or selenium alloy as the recording thin film, and record a signal by changing the state of the thin film between amorphous and crystalline or between two types of crystal structures.
One of the merits of magneto-optical phase change media is that so-called single beam overwriting, wherein a single laser spot erases the old signal as it records the new signal, can be achieved with relative ease (Proceedings of SPIE Vol. 695, pp. 105-109). As shown in FIG. 22 and FIG. 23(a), a new signal can be recorded while erasing the old signal by changing the laser power between two power levels, a recording level and an erase level.
However, according to the prior art recording system, the distortion of the recording mark into a teardrop-shaped mark which results in increased jitter and error rate also occurs, as explained below. When a signal as shown in FIG. 23(a) is used for recording, the achieved temperature of the recording film is relatively low at the front and gradually increases toward the back as shown in FIG. 23(b) due to the effects of preheating. This results in a teardrop-shaped recording mark as shown in FIG. 23(c). The distortion of the recording mark leads to distortion of the playback signal waveform, and is a cause of increased jitter. A number of improved recording systems are proposed for resolving this problem, and are described, for example, in Japanese patent publication (unexamined) Nos. S63-266632 and S63-279431, and in also in U.S. patent application Ser. No. 07/311,362 (corresponding to EP application 89301389.6) which is assigned to the same assignee as the present application.
The proposed systems for reducing recording mark shape distortion, such as disclosed in Japanese patent publication (unexamined) Nos. S63-266632 and S63-279431 are accomplished by composing the recording waveform used to form one recording mark from a pulse string comprising a short pulse of the same shape. In U.S. patent application Ser. No. 07/311,362 (corresponding to EP application 89301389.6), a recording waveform forming a recording mark is converted to a pulse string comprising multiple pulses wherein the interval between pulses is gradually decreased (or the pulse width is gradually increased), or both the recording waveform and the erase beam are modulated by a pulse string, to control the achieved temperature of the recording film and thus reduce the distortion of the recording mark.
Each of the above systems for reducing the distortion of the recording mark in a single beam overwrite system presents specific problems. Those described in Japanese Patent publication (unexamined) Nos. S63-266632 and S63-279431 are achieved with a simple construction, but are nominally effective in improving the recording mark shape.
The invention described in U.S. patent application Ser. No. 07/311,362 (corresponding to EP application 89301389.6) can achieve a large distortion reduction effect, but because it requires pulse strings optimized for pulses of all possible pulse widths in the input signal to be preset, the result is a device of extremely complex construction. In other words, a recording method and recording apparatus using single beam overwriting to form a recording mark with low shape distortion by means of a device of extremely simple construction has not heretofore existed.